In forming aqueous fluoropolymer dispersion, it has long been recognized that small amounts of iron (ferric ions) in the dispersion can lead to a contamination problem called “red scum”. Red scum is the term applied to dispersion which contains a layer of insoluble ferric hydroxide floating on the surface of the dispersion. Iron can get into the dispersion by addition of iron compounds in fluoropolymer recipes or from the metal equipment used in polymerization or processing polymer. In the as-polymerized acidic state, the iron present is soluble. However, in order to prevent bacterial growth, dispersion is usually made basic by the addition of compounds such as ammonium hydroxide or sodium hydroxide. When the dispersion is made basic, the iron becomes insoluble. Eventually the iron particles will float to the surface of the dispersion and appear as a yellow or red oil floating on the surface. This can take several days or weeks. As little as 0.2 ppm of iron can cause visible red scum in dispersion and such scum is an aesthetic problem only, not affecting any of the functional properties of the dispersion. Nonetheless, it has been the practice of the industry to eliminate the scum.
Red scum has been removed by skimming the dispersion with a vacuum tube. This can remove some but not all of the scum. In addition, it is very labor intensive. Preferably the scum has been eliminated by adding a chelating agent for the iron prior to the concentrating of the dispersion. Citric acid has been used as such an agent but will fail to prevent red scum (1) if added after the addition of ammonium hydroxide, (2) if the level of iron is too high for the amount of citric acid added or (3) if other chemicals present complex with citrate and compete with the iron. Alternatively, ethylenediamine tetra-acetic acid (EDTA) was found to be a better chelating agent than citric acid especially for dispersions of FEP which are much more prone to red scum than PTFE dispersions. Scum formation in FEP dispersion appears to be the result of the higher level of fluoride ion content in the FEP copolymer which is due to hydrolysis of the more numerous end groups. In FEP dispersions, the fluoride ion complexes with the chelating agent and thus competes with the iron. The use of EDTA as the chelating agent instead of citric acid results in stronger iron complexing as evidenced by the equilibrium constant for EDTA complex being 1026 as compared to 1011 for citric acid, thereby eliminating the competition presented by the fluoride ion. Such complexes are totally soluble in the dispersion.
As described In U.S. Pat. No. 2,559,752 to Berry, fluorosurfactants are used as non-telogenic dispersing agents in the formation of aqueous fluoropolymer dispersions. As has been further taught in U.S. Pat. No. 3,882,153 (Seki et al) and U.S. Pat. No. 4,282,162 (Kuhls), these expensive fluorosurfactants can be recovered either from the aqueous phase after the polymer has been coagulated from the dispersion or in the aqueous polymer dispersions prior to concentration. A preferred method of recovering the fluorosurfactant from the fluoropolymer dispersion as taught in both Kuhls and Seki et al. is by adsorption onto an ion exchange resin. Strongly basic anionic exchange resin in particular has been found useful for the nearly quantitative removal of the preferred fluorosurfactant, ammonium perfluorooctanoate (PFOA). Ion exchange resins in the hydroxide form are preferred. However, as the fluorosurfactant replaces the hydroxide ion at the ion exchange site and the pH of solution increases, red scum formation is observed for dispersions containing iron.
What is desired is a process to both reduce the amount of fluorosurfactant in the final dispersion product and reduce red scum formation in the preparation of aqueous fluoropolymer dispersions.